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Writing essentials
The Penguin Guide to Punctuation (Penguin Reference Books)The Penguin Guide to Punctuation (Penguin Reference Books) by R L Trask
Buy new: £5.57 / Used from: £2.98
One of the few punctuation guides that makes sense without 10 other books on hand.
The Elements of StyleThe Elements of Style by William Strunk Jr.
Buy new: £5.24 / Used from: £1.07
Read every week until it's a part of you.
Line by Line: How to Edit Your Own WritingLine by Line: How to Edit Your Own Writing
Buy new: £11.50 / Used from: £3.46
Covers everything else.
Self-Editing for Fiction Writers, Second Edition: How to Edit Yourself Into PrintSelf-Editing for Fiction Writers, Second Edition: How to Edit Yourself Into Print by Renni Browne
Buy new: £5.98 / Used from: £5.95
The only fiction how-to I'd reccomend; I read about 20 others and they did more harm to my writing than good.
For Whom the Bell TollsFor Whom the Bell Tolls by Ernest Hemingway
Buy new: £5.47 / Used from: £0.56
Now for some field work :-) Learn more here than in a thousand books on how to write a novel.